Biologically active micro-gradients in cyanobacterial mats of Antarctic lakes and streams

Summary

Microprobe techniques have been employed to examine the micro-gradients present in benthic cyanobacterial mats and sediments of maritime Antarctic lakes and streams. Light penetration was greater in stream mats than in lake mats because of higher incorporation of inorganic material in the latter. Maximum photosynthetic rates were recorded sub-surface in stream mats which are subjected to high levels of solar radiation. An obvious orange-red layer of carotenoids was observed in the top 1 mm of these mats, overlying a blue-green sub-surface zone. Lake mats showed no obvious carotenoid pigmentation and pigment analysis confirmed normal chlorophyll a: carotenoid ratios. This would suggest that the carotenoids have a protective function in stream mats.

Oxygen microprofiles indicated aerobic conditions throughout the stream mat vertical profile. A significant vertical component of lake mats lay beneath the photic zone and was therefore heterotrophic. Oxygen profiles indicated that a chemically reduced zone existed within the mats studied and the use of a nitrate electrode indicated that nitrate was also largely consumed within the mat profile. In situ measurements of oxygen profiles in an ice-covered lake indicated a substantial diffusive boundary layer in excess of 2 mm which would have severely restricted oxygen diffusion into the mats and thus accelerated the process of winter anoxia in these sediments.